Midmonth day (4)
I believe the answer is:
ides
'midmonth day' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ides that I've seen before include "mid-month for the Romans" , "Period in month" , "Caesar's bad day in March" , "date for 21 [ROMAN]?" , "15 March, say -- key date for Caesar" .)